1. Field of the Invention
The present invention relates to apparatus for mounting and fastening printed circuit boards or cards within a rack or chassis, and, more particularly to an apparatus which includes an elongated flexible member that is deflected transversely to fasten the board in place.
2. Description of the Prior Art
Elongated wedge assemblies have been in use for fastening printed circuit (PC) boards within an elongated slot in a rack or chassis. One prior art device assembly includes a center wedge having sloped surfaces at its opposite ends and two end wedges having surfaces that abut against the center wedges' opposite ends. A screw extends lengthwise through the center wedge and connects together the two end wedges. Rotating the screw in one direction draws the two end wedges toward each other, causing them to deflect transversely on the sloped abutting surfaces. This increases the assembly's effective width and therefore fastens the PC board tightly within the slot. Subsequently rotating the screw in the opposite direction moves the two end wedges apart from each other to release the clamping force on the PC board.
Although the wedge assembly described above proved generally effective in locking the PC board in place, its use has not proven to be entirely satisfactory For example, although applying the release torque (rotating screw in the opposite direction), the end wedges do not realign with the center wedge. This occurs because the screw thread is at one wedge end. Tightening compresses the wedges at an approximate 45.degree. angle and forces them outward. The release torque only extends the threaded end wedge axially away from the center wedge. The wedges, although loosened to the extent that the PC board can be removed, do not realign themselves, making installation of the locking wedge assembly and PC board to the chassis track difficult.
The apparatus disclosed in U.S. Pat. No. 4,775,260 to Kecmer overcomes the disadvantages of the prior art elongated wedge assemblies by providing an apparatus which avoids the need for making torque measurements to insure proper installation and which also provides proper alignment of the apparatus' elements during installation. The apparatus includes a center wedge having sloped surfaces at its opposite ends and two end wedges having surfaces that abut against opposite ends of the center wedge, the center wedge being secured to the PC board. A screw extends lengthwise through a channel through the center wedge to connect together the two end wedges. When the screw is rotated in one direction, the two end wedges, together with the screw, are caused to move transversely relative to the center wedge, to increase the effective transverse width of the apparatus and thereby fasten the PC board in place. The screw is associated with clutch means for limiting the maximum forward torque that can be applied to the screw and thus the maximum transverse force the wedges can apply to the PC board. The clutch means disengages when more than a predetermined torque is applied.
Although the wedge assembly described in the aforementioned patent has proven generally effective in locking a PC board in place, there are limitations associated with its use. In particular, the end wedges utilize ramp angles of 45.degree. to provide the wedging or deflecting action. It is well known that the side load force provided by the center wedge is determined by the angle of the sloped surfaces of the center wedge which abut against the sloped surfaces of the end wedges (the shallower the angle with the horizontal, the higher the side loads). Thus, in order to increase the side loads provided by the apparatus disclosed in the Kecmer patent, the angle of the sloped surface of the center wedge with respect to the horizontal would have to be decreased. In other words, the angle would have to be much shallower than the 45.degree. angle shown in order to provide the necessary mechanical advantage. The problem is that the slope becomes so large that on shorter pieces a spacing problem would arise. In addition, making the screw smaller to reduce the size of the wedge apparatus would increase torquing problems. Thus, for small circuit boards and closer pitched spacing, it is difficult to modify the Kecmer apparatus. In the age of miniaturization where circuit boards and spacings are becoming smaller and thus smaller sized wedge assemblies having the capability of providing high side loads are required, this factor limits the utilization of the Kecmer apparatus. In essence, the apparatus shown in the Kecmer patent has a minimum size below which modification thereof would be ineffective. Even if the apparatus could be reduced in length to accommodate small size circuit boards and thus shallower spacing, the amount of side load available would be severely limited.
U.S. Pat. No. 4,823,951 to Colomins describes a locking wedge assembly similar to that shown in U.S. Pat. No. 4,775,260 and thus has similar disadvantages regarding size reduction and side load capability.
What is thus desired is to provide a simple and cost effective printed circuit board mounting apparatus which is of a smaller size than apparatus currently available and which, despite its smaller size, provides relatively high side loads.